Well, that's not unique to IB. I understand many, if not most, forex dealers, offer that capability, as long as your margin is sufficient. For instance, Oanda's published limit is 10M, but nothing stops you from entering more than one (single currency pair) order at that size, at the same price. They'll all execute simultaneously. And you can exit them all at once with a single click, if you want to, or modify all the stops or limits by dragging the mouse.
Second, without the benefit of a front-end like BT, attempting to simultaneously enter, manage and exit more than one large order per currency pair in IB, whether in one account or across more than one account, is slow, repetitive and cumbersome. To be fair, that's obviously something very few people would care about; if you do, you know what to do (see Steve_IB's post above).
